diff --git a/admin.info b/admin.info
index 84dce14..c56a0ed 100644
--- a/admin.info
+++ b/admin.info
@@ -3,3 +3,5 @@ name = "Admin"
 description = "UI helpers for Drupal admins and managers."
 package = "Administration"
 core = 6.x
+; array_diff_key() requires php 5.1+
+php = 5.1.0